The E4000 Tuner offers a stepping stone into the world of SDRs. It does the heavy lifting,
significantly reducing the size, cost, and complexity of a SDR. It does not do any
programmable signal processing, which is where SDRs are strongest. Quantization has been
left out of the discussion because the RTL2832 is so bad at it (to conserve size and
cost). The RTL2832 hamstrings the usefulness of the E4000 significantly narrowing the
application of DSP. If you want high fidelity reception, you must apply a high fidelity
ADC. The Tuner Dongle offers a low cost opportunity for an enterprising hacker to jump
ahead and produce a product worth pursuing. As is it is very limited.
Also, GSM is very narrowband voice. Data is wideband and complex, Data is where the Gold
is, mining it takes better tools which the RTL2832 prevents application to the reception.
Typical low price ADC ...The ADC12DS105 is a dual 12-Bit, 105 Msps A/D Converter
http://www.ti.com/product/adc12ds105 Price $31
http://www.ti.com/tool/adc12ds105lfeb Design Kit $495, Accessories include a USB
Interface
http://www.ti.com/solution/software-defined-radio-sdr-diagram
http://www.ti.com/lit/an/slaa407/slaa407.pdf
How to Select the (Best Minimum) Sampling Rate
The selection of sampling rate is also important for ADCs. Fast encode clocks offer the
advantage of
making analog filtering significantly easier. Given a fixed signal bandwidth, a higher
encode rate increases
the allowable transition band. This allows lower order filters to be used to reduce cost,
or, if higher order
filters are still used, greater stop band rejection can potentially be realized.
Another advantage with increasing clock speeds is processing gain. Processing gain is
achieved when the
signal of interest is oversampled and then digitally filtered. Processing gain occurs when
noise outside the
band of interest is digitally removed, which results in improved in-band SNR. Processing
gain can be
easily determined using Equation 1. This equation determines the increase in SNR over and
above the
SNR of the converter alone.
Processing Gain = 10log10 (Half of the sample rate/ filter BW)
Also, when selecting the sampling rate, the user must consider the second and third
harmonics (HD2 and
HD3). These harmonics must lie outside the band of interest. These can be filtered using
the digital filter
inside the DDC/DSP. That means, frequency planning also becomes important when designing a
SDR
system.
-----Original Message-----
From: osmocom-sdr-bounces(a)lists.osmocom.org [mailto:osmocom-sdr-bounces@lists.osmocom.org]
On Behalf Of Sylvain Munaut
Sent: Sunday, July 08, 2012 10:29 AM
Cc: osmocom-sdr(a)lists.osmocom.org
Subject: Re: stroboscopic (aka equivalent time sampling) using EZCAP DVB dongle ?
1) Yes, if all you need is to be able to detect a
signal on an analog stream its Nyquist Frequency will do, (2 times the frequency being
measured) a 20 megasample ADC would be adequate for the E4000 tuner's I/Q output.
This is IQ signal you don't need twice the bandwidth, you only need once. So a 8 Msps
IQ would be adequate to represent a 8 MHz bandwidth.
[snip ....]
You need at least 10 times that bandwidth to accurately reconstruct Phase Modulated
Signals.
No you don't. You want a concrete example: Most GSM phone are demodulating a phase
modulated signal by sampling at exactly one sample per symbol (so they're even
sampling _lower_ than the actual signal bandwidth)
And although oversampling is useful in certain case to analyze signal, it's
essentially useless if the signal has _already_ been filtered like it is in the E4000
case. If the IQ output has only 8 MHz of useful content at the output, you can oversample
all you want, it won't bring back what the E4000 discarded ... Those antialiasing
filter you mention, they're in the E4000 !
2) I didn't say bigger / better SDR would be useless ... I'm just saying if
you're spending hundreds / thousands on better ADC and better PC interface you might
as well ditch the E4000 and use a better RF front end as well. The spectral output of the
E4000 is far from being "clean", there is distortions / LO leakage / random
spurs / 28.8M clock leakage / ... etc ... Oh and Elonics declared bankrupcy so the E4000
supply is fading ...
Cheers,
Sylvain